Heating device

ABSTRACT

A heating device for producing graphic images on sheet material. The device includes a novel travels platen that provides an even amount of heat over the whole heating surface of the heating platen. The heating platen includes a heat source, an exterior heat conductive plate having on one side a heating surface adapted to contact and conduct heat to sheet material, and a sheetlike spacer between the heat source and exterior heatconductive plate that conducts heat more rapidly along its length than through its thickness and thereby spreads heat from the heat source as it travels to the exterior heat-conductive plate.

United States Patent Inventor John A. Svendsen St. Paul, Minn. Appl. No. 888,412 Filed Dec. 29, 1969 Patented Dec. 21, 1971 Assignee Minnesota Mining and Manufacturing Company St. Paul, Minn.

HEATING DEVICE 10 Claims, I Drawing Fig.

U.S.Cl 219/216, 165/185, 21 /388, 219/469 Int. Cl 1105b 1/00 Field of Search 219/216,

388, 469-471; 100/93 P, 93 RP; 356/651, 65.2; 355 9, 1 1; 156/583; 263/6; 165/185 Primary Examiner-C L. Albritton A!lorneyl(inney, Alexander, Sell, Steldt & Delahunt ABSTRACT: A heating device for producing graphic images on sheet material. The device includes a novel travels platen that provides an even amount of heat over the whole heating surface of the heating platen. The heating platen includes a heat source, an exterior heat conductive plate having on one side a heating surface adapted to contact and conduct heat to sheet material, and a sheetlike spacer between the heat source and exterior heat-conductive plate that conducts heat more rapidly along its length than through its thickness and thereby spreads heat from the heat source as it travels to the exterior heat-conductive plate.

PATENIH] nan-21 m lNVliN'l ()R. JOHN A. aS I END SEN mawa A 7' TORNE Y8 HEATING DEVICE BACKGROUND OF THE INVENTION Several processes for forming images on sheet material require heat-either applied to the sheet material itself to develop a latent image or applied to sheet assemblies of the sheet material and either the original or an intermediate sheet to transfer the image to the sheet material. One generally useful device for providing the necessary heat comprises a heating platen in the form of an arcuate heated shoe and a pressure platen in the form of a rotatable cylindrical drive roll against which the shoe is pressed. The arcuate heated shoe comprises a thin arcuate heat-conductive plate having a thin heating blanket-formed from a grid of electric conductors in an insulating sheath-bonded to the back side of the plate. On its front side the heat-conductive plate is provided with a heating surface adapted to contact the sheet material or sheet assemblies.

While the described device is useful to rapidly provide heat to sheet material or sheet assemblies, the heat provided has sometimes been found to vary over the heating surface of the heating platen. This nonuniformity of heat can be traced to the fact that the heat from the heating blanket originates in finite, separated electric conductors, and to the fact that at junctions of conductors, and in places where the insulation of the heating blanket is thinner than in other places, the heating blanket develops hot spots. Since the intensity of an image formed on sheet material may be dependent on the amount of heat reaching the sheet material or sheet assembly, an image that is uneven in intensity sometimes is produced. This uneven intensity sometimes lessens the quality of a copy of printed matter by making portions difficult to read or even unintellegible. But uneven intensity is especially critical and intolerable when continuous-tone images are being produced. In order for an image to have continuous-tone quality, the intensity of the image" must be essentially uniform over its whole area, and prior art heating platens as described above have not reliably provided the needed degree of uniformity of heat over their heating surface.

SUMMARY OF THE INVENTION The present invention provides a heating device having a novel heating platen that is capable of evenly heating the whole area of sheet material or sheet assemblies placed against the platen. This new heating platen comprises as a laminate structure (a) an exterior heat-conductive plate having on one side a heating surface adapted to contact and conduct heat to the sheet material or sheet assembly, (b) a heat source substantially coextensive with and parallel to the heating surface of the exterior heat-conductive plate, and (c) a sheetlike spacer between the exterior heat-conductive plate and heat source in position to conduct heat between the heat source and the exterior heat-conductive plate. The spacer is made from a material such as paper that causes the spacer to conduct heat more rapidly along its length than through its thickness. The result is that heat from hot areas of the heat source is spread as it travels to the heating surface of the platen, whereupon the heat becomes evenly distributed over the whole area of the heating surface.

A heating device of this invention further includes a pressure platen that presses sheet material against the heating surface of the heating platen. Preferably, the pressing surface of the pressure platen is covered with a self-sustaining resilient foam to prevent drainage of heat through the pressure platen and to provide a uniform pressure against the heating surface of the heating platen. Heating devices of the present invention, with their uniform application of heat and uniform pressure against the heating surface provide continuous-tone images of high quality.

DESCRIPTION OF THE DRAWING The drawing is a partial section through an illustrative heating device of this invention.

DETAILED DESCRIPTION The illustrative heating device 10 of the invention shown in the drawing comprises an arcuate heating platen 11 and a rotatable cylindrical drive roll 12 that draws sheet material between it and the heating platen and serves as a pressure platen pressing the sheet material against the heating platen. The heating platen 11 includes several different elements as a laminate structure. A base heat-conductive plate 15 has a heat source in the form of a heating blanket l6 bonded over its back surface. The heating blanket 16 comprises a grid of electric conductors l7 sheathed in an organic electrical insulating material 18. Fibrous heat insulation 19 and an exterior housing 20 cover the heating blanket to conserve heat and protect users against contact with the hot blanket. The illustrative heating platen is completed by a sheetlike heat-spreading spacer 22 and an exterior heat-conductive plate 23, with the heat-spreading spacer sandwiched between the base and exterior heat-conductive plates.

The heat-spreading spacer can be made from several different materials that conduct heat more rapidly along their length than through their thickness. In general, the spacer should conduct heat at least twice as fast along its length as through its thickness. Paper, which is formed from fibers that lie in the plane of the paper, is a preferred material since it is inexpensive and quite excellent in spreading capabilities-it often conducts heat five or six times faster along its length than through its thickness. Alternatives to paper include thin sheets of mica paper or asbestos, and less desirably, waffleconfigured thin metal foils and very fine wires screens. While spreading heat, the spacer must still conduct heat rather rapidly between the heating blanket and the exterior heating plate to provide sufficient heat to overcome the heat withdrawn by sheet material passed over the heating surface. The heat-conductive base and exterior plates typically conduct heat more rapidly than the spacer and are usually a metal such as aluminum, brass, or copper.

The cylindrical drive roll 12 of the embodiment of the invention illustrated in the drawing includes a preferably heatinsulative central tube 25 covered with a layer 26 of selfsustaining resilient foam. Instead of being covered with a selfsustaining foam, the pressure platen may be covered with a layer of flocking or furlike fabrics. The foam, such as a polyurethane foam, is preferred principally because it provides the most uniform pressure on sheet material, thus avoiding mottle patterns and other kinds of uneven development. In addition, foam is a better and more uniform insulator, and it develops the greatest drive force on sheet material. When a copy sheet 27 is drawn into the heating device in the direction of the arrow 28, the layer of resilient foam 26 collapses somewhat to accommodate the copy sheet.

The exterior heating surface 230 of the heat-conductive plate 23 is a low-friction, low-adherence surface with respect to the surface of sheet material passed over it. Preferably the surface 230 is covered with a low-friction material such as polytetrafluoroethylene. Some sensitized sheet material is best developed when a functional layer of image-forming material on the sheet material is directly against the heating surface of the heating platen, and a polytetrafluoroethylene coating has been found to provide a good contact surface against the image-forming coating on such sheet material.

Prior art, arcuate heating platens generally comprise a structure like that shown in the drawing except that they do not have the spacer 22 and exterior plate 23. Such prior art heating platens may be conveniently modified in the fleld by feeding between a drive roll such as the drive roll 12 and existing heating platen a sandwich of paper or other heat-spreading spacer and heat conductive smooth-surfaced plate. The added plate desirably has some means like a curved end flange such as the flange 23b on plate 23 which catches on the existing structure and prevents the sandwich from being completely fed through the device.

Instead of taking the form of an arcuate shoe such as shown in the drawing, heating platens of the invention also comprise a heated rotatable cylindrical roll, in which case the pressure platen is usually an arcuate shoe. Heating platens of the invention also have a flat or gently convexly curved heating surface, in which case they may be used with pressure platens that have a cooperating structure or with pressure platens in the form of rollers that are drawn across the heating surface of the heating platen.

Although the embodiment of the invention illustrated in the drawing includes a base heat-conductive plate to which the heating blanket is attached, heating platens of the invention also are formed by omitting the base heat-conductive plate and directly attaching the heating blanket to a heat-spreading spacer. In such a device it may be desirable to increase the thickness of the exterior heat-conductive plate to accomplish a greater heat spreading by that plate.

As an example of the invention, a heating platen such as illustrated in the drawing was formed using as the base heatconductive plate a sheet of aluminum 35 mils thick, on one side of which a heating blanket that comprised a network of nichrome resistance heating wires encased in silicone rubber was attached. A sheet of photographic base paper, 6% mils thick, was disposed between the base heat-conductive plate and an exterior heat-conductive aluminum plate that was 32 mils thick. On the exterior surface of the exterior heatconductive plate was a coating 0.7-mil-thick of polytetrafluoroethylene. The drive roll comprised a EPA-inchdiameter tube of paper-filled phenolic resin having a wallthickness of one-eighth inch and covered with a ,fi-inch-thick layer of polyurethane foam.

lclaim:

l. A heating device for producing graphic images on sheet material comprising 1. a heating platen that comprises as a laminate structure (a) an exterior heat-conductive plate having on one side a heating surface adapted to contact and conduct heat to the sheet material, (b) a heating blanket that is on the other side of the exterior heat-conductive plate and is substantially coextensive with and parallel to the heating surface of the exterior heat-conductive plate, and (c) a sheetlike spacer between the exterior heat-conductive plate and heating blanket in position to conduct heat between the heating blanket and the exterior heat-conductive plate, the spacer conducting heat more rapidly along its length than through its thickness; and

2. a pressure platen adapted to press sheet material toward the heating surface of the exterior heat conductive plate.

2. A heating device of claim 1 in which the sheetlike spacer is a fibrous paper.

3. A heating device of claim 1 in which the surface of the pressure platen that presses against the sheet material is covered with a uniform layer of self-sustaining resilient foam.

4. A heating device of claim 1 in which the heating platen is arcuate and the pressure platen is a rotatable cylindrical roll that mates within the concave arc of the heating platen.

5. A heating device of claim I in which the heating platen further includes a base heat-conductive plate between the spacer and the heating blanket.

6. A heating device of claim 1 in which a low-friction lowadherence coating covers the heating surface of the exterior heat-conductive plate.

7. A heating device for producing graphic images on sheet material comprising 1. an arcuate heating platen that comprises as a laminate structure (a) an arcuate base heat-conductive metal plate, (b) an arcuate exterior heat-conductive metal plate having on one side a smooth heating surface covered with a low-friction coating and adapted to contact and conduct heat to sheet material, (c) a heating blanket comprising a grid of electric conductors sheathed in electrically insulative organic polymeric material, the blanket being bonded to the convexly curved surface of the base heat-conductive plate and positioned substantially coextensive with and parallel to the heating surface of the exterior heat-conductive plate, (d) heating insulation covering the nonbonded surface of the heating blanket, and (e) a sheetlike spacer between the base and exterior heatconductive plates and in position to conduct heat from the base heat-conductive plate to the exterior heat-conductive plate, the spacer conducting heat more rapidly along its length than through its thickness; and

2. a rotatable drive roll that mates within the arc of the heating platen, is covered over its exterior surface with a layer of resilient heat-insulative material, and is adapted to press sheet material toward the heating surface of the exterior heat-conductive plate.

8. A heating device of claim 7 in which the sheetlike spacer is a fibrous paper.

9. A heating device of claim 7 in which the resilient heat-insulative material covering the drive roll is a resilient selfsustaining foam.

10. A heating device of claim 7 in which the spacer and exterior heat-conductive plate are not attached to the base heatconductive plate and are removable from between the base heat-conductive plate and the cylindrical roll.

t i i i 

1. A heating device for producing graphic images on sheet material comprising
 1. a heating platen that comprises as a laminate structure (a) an exterior heat-conductive plate having on one side a heating surface adapted to contact and conduct heat to the sheet material, (b) a heating blanket that is on the other side of the exterior heat-conductive plate and is substantially coextensive with and parallel to the heating surface of the exterior heat-conductive plate, and (c) a sheetlike spacer between the exterior heat-conductive plate and heating blanket in position to conduct heat between the heating blanket and the exterior heat-conductive plate, the spacer conducting heat more rapidly along its length than through its thickness; and
 2. a pressure platen adapted to press sheet maTerial toward the heating surface of the exterior heat conductive plate.
 2. a pressure platen adapted to press sheet maTerial toward the heating surface of the exterior heat conductive plate.
 2. A heating device of claim 1 in which the sheetlike spacer is a fibrous paper.
 2. a rotatable drive roll that mates within the arc of the heating platen, is covered over its exterior surface with a layer of resilient heat-insulative material, and is adapted to press sheet material toward the heating surface of the exterior heat-conductive plate.
 3. A heating device of claim 1 in which the surface of the pressure platen that presses against the sheet material is covered with a uniform layer of self-sustaining resilient foam.
 4. A heating device of claim 1 in which the heating platen is arcuate and the pressure platen is a rotatable cylindrical roll that mates within the concave arc of the heating platen.
 5. A heating device of claim 1 in which the heating platen further includes a base heat-conductive plate between the spacer and the heating blanket.
 6. A heating device of claim 1 in which a low-friction low-adherence coating covers the heating surface of the exterior heat-conductive plate.
 7. A heating device for producing graphic images on sheet material comprising
 8. A heating device of claim 7 in which the sheetlike spacer is a fibrous paper.
 9. A heating device of claim 7 in which the resilient heat-insulative material covering the drive roll is a resilient self-sustaining foam.
 10. A heating device of claim 7 in which the spacer and exterior heat-conductive plate are not attached to the base heat-conductive plate and are removable from between the base heat-conductive plate and the cylindrical roll. 